When a zipper strip is applied to a film or web, heat and pressure are applied to effect the welding of the zipper to the film or web. The heat and/or the pressure can result in damage to the closure. Various measures have been adopted to try to minimise the effects of the heat and pressure. For example, the welding can be of flanges extending laterally from the reclosable male and female elements, to try to minimise damage to the closure. However, it can still happen that the zipper is damaged or distorted as a result of these external influences.
Our international patent application WO-A-02/04298 describes a novel zipper whose design is resistant to distortion or damage due to the welding heat and/or pressure.
In that application there is described a reclosable fastener for plastic bags and other containers comprising two elements, each element comprising at least one hook engageable with a hook of the other element, and each element comprising an upstanding post at the margin of the fastener which is engageable with a heel of the other element at the opposing margin of said other element, wherein the respective posts and heels of the two elements are angled at their respective contact surfaces.
The fact that the respective contact surfaces of the posts and heels are angled or mitred enables the closure more easily to resist pressure without distortion and without the hooks being squashed. The margins of the closure are more easily able to resist bending and to maintain their desired supporting function.